Advanced Electrochemical Strategies for Optimizing the Electrodeposition of Nickel, Zinc, and Zinc-Nickel Alloys

Applications of electrodeposited nickel

Nickel is the most common electrodeposited metal among the metals: Zn, Cd, Cr, Cu, Au, Sn, and Ag, and their alloys (Sn-Zn, Zn-Ni, brass, and bronze). Electrodeposited Ni improves appearance, durability, and performance in various industrial and engineering applications.^[1,2] Examples include functional and decorative coatings for jewelry, magnets, micro-electromechanical systems, electrocatalysts for hydrogen production, and the co-deposition of alloys or composite layers.^[3-5]

A nickel metallic coating protects steel, aluminum, copper, and stainless steel (St.St.) against erosion corrosion by filling surface pores with a uniform layer of a specified thickness, known as the throwing power (T.P.). Black Ni is used for decorative purposes, as a photothermal-absorbing conversion material, and in the aerospace and defense industries.^[6] Nickel coating is a good alternative to Cr-based coatings due to its hardness, excellent corrosion and wear resistance, self-lubricating ability, and thermal stability.^[7] It is also used as an anode for Li-air batteries^[8] and as a coating on chips in protein microarray fabrication technologies.^[9] Due to their mechanical strength, Ni deposits are utilized in various applications, including printing, phonography, foils, tubes, electroformed stampers, dies, molds, and numerous other technologies.^[10,11]

Ni resists many corrosive media, especially alkali. Ni and Ni alloys are used in the storage of caustic NaOH solution. Corrosion resistance is proportional to Ni content. Most corrosive caustic solutions are handled in cast iron alloy (2% Ni), which increases mechanical strength and offers outstanding high-temperature oxidation and corrosion resistance in many corrosive environments. Stainless steel (St.St.), an alloy containing about 10% Ni, resists stress-corrosion cracking (SCC). Ni alloys are passive and contain varying weight percentages of alloying elements. Inconel (76Ni, 0.08 C, 16Cr, 8Fe) resists SCC.^[12] Dure Ni (94Ni, 0.15C, 4.5Al, 0.5Ti) has high mechanical strength and corrosion resistance; Monel (63-66Ni, 0.15C, 29-31Cu) is used for storing hydrofluoric acid. Hastelloy C (56 Ni, 0.08 C, 15 Cr, 17 Mo, 5Fe) and Hastelloy B (62 Ni, 0.10 C, 28 Mo, 5Fe) alloys are the most corrosion-resistant alloys. The commercially available Nichrome (80 Ni, 10 Cr) is used in the electrical resistor heater. Ni and high-Ni alloys even contain substantial amounts (8%Ni) and resist the most severe corrosive media. Ni resists corrosion in the neutral and slightly acidic solutions in the food industry.^[13] Ni and Ni alloys corrode in strong oxidizing solutions, such as HNO₃ and NH₃ solutions, as well as sulfur-bearing gases at elevated temperatures, through embrittlement^[14,15] due to the formation of intergranular, low-melting-point (645°C) porous, defective, non-protective sulfide eutectic scales.^[16] Ni alloys are more corrosion resistant, especially to pitting corrosion and SCC, than St.St. alloy series 300.^[17]

Zinc ED

It is the most widely applied electrochemical technique in various sectors of the automotive industry and technologies.^[18,19] Zn and Zn-alloy anticorrosive coatings extend the lifespan of critical mechanical metallic components.^[20] Zn is the perfect sacrificial anode for cathodic protection of C-steel alloy. The low protection efficiency in aggressive acids is due to active behavior.^[21-23] Zn-protected steel (pipes, fences, nails, etc.) resists corrosion by forming a white conversion coating film. Electroplated Zn sheets on exposed automobile body panels provide a better, more uniform, and smoother coating with a thick range of 4 μm to 14 μm, compared to hot-dipping galvanization.^[24]

Zinc-nickel alloys

ED Zn-Ni alloy coatings are extensively applied in aerospace, oil, gas, and automotive industries. The excellent corrosion resistance, ductility (permitting plastic deformation without fracture for unlimited fabrications), low hydrogen embrittlement, and good electrical, electronic, and thermal conductivity^[25-29] are attributed to the non-directional metallic bonding between Zn atoms.^[30] The electroplating efficiency depends on the type of additives in the electroplating bath, which control the deposition rate, crystallization mechanism, grain refining, compactness, leveling, and brightness.^[31,32]

Ni ED from aqueous Watt bath additives electrolyte

Ascorbic acid (Asc.), an eco-friendly additive, improved corrosion resistance.^[33] A high concentration of ascorbic acid caused a cathodic shift in the polarization curves (current density J vs. potential E; Figure 1) and inhibited the reduction of Ni(II) ions. Tafel behavior obeyed equation 1:

Close

Figure 1:

Polarization curves ED Ni on Cu at 27°C, different [Asc.]: redesigned with permission CCBY 4.0 (2022), ref. 33. High [Asc.] and pH affect the required potential for ED.

Export to PPT

Exchange current densities (J₀) were obtained by extrapolating the Tafel lines to 0.0 V overvoltage.

Where b and αc are the cathodic Tafel slope and the charge-transfer coefficient (the fractional activation overvoltage for reduction), respectively.

The fraction (1-αc) represents the activation overvoltage for oxidation. Figure 2 shows the relationship between the overpotential and the current density.

Close

Figure 2:

Represented Tafel equation. (Constants a, b characterize each electrode process), b equals 30 mV-140 mV, depending mainly on the cathode material and slightly on the ED mechanism. In the hydrogen evolution reaction (HER) during ED, Ni or Zn, b =120 mV, and the discharge (H⁺+ e→ H) process is the rate-determining step (RDS). For a smooth Pt. cathode, b =30 mV, and the combination (H+H →H_2(g)) is RDS.

Export to PPT

Increasing the concentration of Asc. Acid decreased the overpotential b, α, Jo. High pH decreased both J_o and b but did not affect αc. Adsorbed molecules formed a coordinate bond with the Ni surface via electrons (es) transfer to vacant d-orbitals. The adsorption data followed the Langmuir adsorption isotherm. The ED mechanism was diffusion-controlled crystal nucleation and growth. A high T.P. at 0.4 M Asc. Increased the microhardness, refined the deposit grains, and improved resistance to uniform and pitting corrosion in NaCl. Computational simulations explored the reduction mechanism of Ni(II) ion during electroplating, as the ascorbate ion formed a stable Ni complex.^[33]

The natural kermes dye (NKD) with high oxygen content enhanced corrosion resistance and microhardness; it also shifted the polarization curve and plating potential in the noble direction, thereby catalyzing Ni(II) ED. Relatively constant αc values suggested the exact deposition mechanism. The dye slightly affected the cathodic current efficiency (CCE) and T.P. The pXRD patterns indicated that the dye did not change the preferential orientation of the Ni crystal planes.^[34]

The amino acid (aa) L-proline refined the Ni deposit grains, shifted the polarization curve and deposition potential to more noble values, and catalyzed the reduction of Ni(II) ions. The value αc suggested the exact deposition mechanism. J-time chronoamperometry transients confirmed Ni deposition as an instantaneous nucleation process followed by 3D growth under charge transfer control. The aa decreased both T.P. and corrosion resistance. Kinetics and mechanism require further investigation.^[35] At low additive concentrations, a bright, hard, and adherent Ni coating was produced with high CCE using simple IL non-volatile electrolytes like pyridinium, imidazolium, and phosphonium halides. ILs are ionic conductors, chemically and thermally stable at room temperature (RT), eliminating the need for a conventional aqueous electrolyte bath, simplifying ED, and enhancing coating quality.^[36]

Molecular additives (nicotinic acid (NA) and CH₃-nicotinate (MN), 5, 5-di-CH₃-hydantoin (DMH), and boric acid (BA)) slightly impacted the bulk speciation of Ni²⁺ ions but significantly affected the electrochemistry of Ni deposition and stripping. Low [NA, MN] altered crystal nucleation and growth mechanisms, forming bright, smooth, and adherent coatings instead of dull, rough traditional coatings. Additives: slightly reduced CCE, catalyzed deposition rates, controlled deposit morphology by direct crystal growth, and guided Ni crystallite growth in the crystal planes (111), (311), and (220) orientations, respectively. The MN was the most significant hardener.^[37]

In Ni ED on Ni substrates without additives, the current density (J) affects grain shape and size, surface appearance, crystal orientation, distribution, and nodule size by changing the growth mechanism. Low J affected the surface characteristics of the fine grains. Effective ED displayed pyramidal-shaped crystallites surrounded by finer grains and nodules.^[38]

In Ni ED from slightly acidic gluconate solutions containing sulfate (SO₄²⁻) and/or chloride (Cl⁻) ions.^[38] Halides Ni complexes strongly affect cathodic reactions at -1.3V. At more negative potentials, ED is influenced by the release of Ni(II) ions from the Ni-gluconate complex in slow RDS. Different species affected nucleation, morphology, and the deposit’s structure. Strongly bonded Cl^- ion refined ED grains and inhibited the growth of the crystal planes. The rapid reduction of NiCl⁺ favored 2D/3D instantaneous nucleation. Large SO₄^-2 species hindered nucleation, forming a larger polyhedral Ni crystal. The NiGlu⁺ is deposited below -1.4V, producing thin and fine-grain deposits.^[39]

The wettability of coating layers correlated with the structural surface morphology of the steel object and its corrosion resistance. The pXRD peaks confirmed (111), (200), and (220) Ni phases. Wetting of a liquid drop on a flat, smooth, chemically homogeneous coating surface characterized by the wetting contact angle, which is calculated using equation 2. The theta value is less than 90° (hydrophilic), greater than 90° (hydrophobic), or larger than 150° (superhydrophobic).

Where $\gamma SV,$ $\gamma SL,\gamma LV$ are solid/vapor, solid/liquid, and liquid/vapor interfacial energies, respectively. The term $\gamma LV$ is the only experimentally measured parameter. Surrounding molecules attract bulk liquid molecules in all directions. Other molecules (energetically unfavorable) attract surface molecules inward. Interfacial energy is consumed in molecular motion from the bulk liquid to the liquid surface to form a new surface.

For $\gamma SV>\gamma SL$, the contact angle θ ranges from 0° to 90°, indicating that the liquid partially wets the hydrophilic surface. Angles above 90° signify a non-wetting (hydrophobic) surface. Ni-rhenium (Re) alloy coatings ED from Watts-type electrolytes. Optimized ED variables affected coating quality, chemical composition, J, and Faradaic efficiency (FE). Microstructure, phases, roughness, microhardness, and abrasion resistance were evaluated after ED at optimal deposition variables: 5 mA∙cm^–2, pH 5.5, and 60-70°C. A bright Ni-rich alloy (Ni₈₀Re₂₀) coating with FE 99.5% contained 43 Wt. % Re, fcc-Ni(Re) and hcp-Re phases; reflectivity 44% with low surface roughness 9.17 nm obtained at optimum deposition variables. Surface quality, microhardness, and abrasion resistance improved by 20 wt.%.^[40]

Ni ED on glassy carbon from acidic baths with various compositions. Cyclic voltammetry and potentiostatic measurements revealed that the cathodic process was hindered by sulfate and gluconate ions. In addition, it improved by increasing the buffer capacity of the solutions. Chronoamperometry revealed that Ni nucleation followed a progressive mechanism, with simultaneous proton reduction occurring during the crystal growth. Simulated nucleation is applied in parameter calculations: nucleation rate, effective diffusion coefficient (D) of Ni species; density of nucleation sites, rate constant of proton reduction, and Ni(II) ion distribution in bulk solutions. The morphology of deposits is significantly influenced by deposition potential and gluconate ions rather than anion type.^[41]

Glycine amino acid in ChCl-urea ionic liquid (IL), affected the ED mechanism of Ni-Mn alloy film as well as its composition, microstructure, and film properties. Cyclic voltammograms revealed that glycine inhibited the reduction of Ni⁺² ions, promoted the decrease of Mn⁺² ions during the cathodic scan, and facilitated the dissolution of both metallic coatings during the reverse scan, thereby altering the mode of film growth. Mn content increased at high glycine concentration and J value. Mn (3.1 Wt.%) minimized the J value to 3×10⁻⁷ A∙cm^-2 and exhibited better corrosion resistance than the Ni film in 3.5 wt.% NaCl solution.^[42]

Ni ED on AISI 1018 steel using quaternary NH₄-sulfate salts (QAS) at various J. The Low J value formed TF fine-grain coatings. Nyquist impedance plots showed a large charge transfer resistance (Rct) semicircle with two-times constants that simulated the Ni coating film and the formed electrical double layer (EDL) at the steel/Ni interface.^[43] Decreased EDL capacitance by QAS reflected an adherent, thick protective coating with characteristics resembling those of an EDL insulator capacitor. Additive compositions significantly influenced Bode plots and phase angle theta (θ^o). The impedance Z decreased with increasing frequency, indicating a dielectric-insulated coating film, which was confirmed by θ_max. at 80^o. Aprotic salts: A tertiary methyl-substituted six-carbon alkyl chain displayed superior corrosion resistance. Hexadecyl alkyl carbon chain weakly adsorbs due to the inductive and steric hindrance.^[43]

Thiourea (TU) affected Ni ED from acidic sulfate solutions over two hours at 60°C: an increase in TU concentration [TU] from 2.0 to 40.0 mg∙dm³ slightly decreased CCE by 3-4%. Higher [TU] deteriorated Ni deposits, weakened surface morphology, contaminated and affected crystallinity. Cyclic voltammetry at 25°C revealed that TU is a cathodic-type inhibitor at 10 mg dm^-3 and it is a mixed-type inhibitor at higher concentrations, affecting J values.^[44] The synergistic effect of commercial additives at various concentrations and deposition times influenced the properties of ED Ni on the steel surface. Using a Hull cell and following a 25-1 fractional design enabled evaluation of brightness, roughness, and charge transfer resistance (R_ct).^[44] Several synergistic and antagonist interactions among factors are discussed physically. Principal component analysis (PCA) indicated that clustering in the PC space corresponds to characteristic morphology.^[45]

Ni coatings ED on Q235 steel using reverse pulsed ED in NiSO₄-0, 0.1, 0.2, and 0.3 g/L phytic acid (PA) additive that affected microstructure and morphology, as the microscopic surface analysis confirmed. PA affected corrosion resistance and induced nm twins (NT) in the grains’ interior due to the lowering of stacking fault energies of Ni during ED, resulting in typical morphological pyramidal surface islands. Its effect was not monotonous with increasing concentration: 0.2 g∙L^-1 improved passivation and maximized the corrosion resistance.^[46]

The steady-state nucleation rate of Ni increased at high concentrations of valine that adsorbed on the ED Ni, forming the Ni-Valine complex on the GC electrode. High pH retarded adsorption by forming Ni(OH)₂. Ni electrocrystallization followed a 3D progressive nucleation mechanism irrespective of pH or concentration of valine.^[47] The initial Ni ED phase on polycrystalline copper cathode in the presence of the serine amino acid followed an instantaneous nucleation mechanism. Stable Ni clusters grown under limited diffusion conditions. HER interference confirmed from partial curves of both Ni deposition and HER. Kinetic parameters of nucleation, including diffusion coefficient of Ni(II) ions in solution, active sites density, nucleation rate, Gibbs free energy (ΔG_{critical nucleation}), and nucleus size, were calculated.^[48] The electrochemical behavior and catalytic activity of ED NiOx are enhanced by electrochemical activation in an aqueous alkali electrolyte. Sensing traces of L-glutamate was improved by introducing an NH₂ group from immobilized 3-aminopropyl-triethoxysilane, creating a positively charge surface. The synergism of NiOx and NH₂ accurately achieved high precision and sensitivity in the sensor.^[49]

In the ED of Ni and Ni alloys, the coating’s microstructure, internal stress, and properties are controlled by the chemical composition of the bath. Pulsed ED and the use of citrate baths^[2,3] obtain high-performance bulk nanocrystalline coatings having fundamental mechanical and chemical properties. The organic and natural additives refine the coating surface. Additives such as gluconate,^[5,41] ascorbic acid, L-proline, kermes dye,^[33-35] and amino acids valine, serine^[47,48] influence the kinetics of Ni electrocrystallization, microstructure, and the brightness. These modifiers act as complex agents or surface-active agents, controlling the crystal nucleation and growth.^[37,46]

For the film properties, the Ni coatings exhibit residual stress gradients,^[4] which are crucial for microelectronic applications. Their morphology is highly dependent on parameters like current density.^[38] Sustainable electrolytes, such as Deep Eutectic Solvents (DESs), improve the brightness.^[36] The electroforming process enables the application of Ni in micro- and macro-manufacturing.^[11] Ni alloying and composites with other elements or materials are functionalized coatings that resist wear or have catalytic properties. NiMo alloys are an excellent corrosion-resistant coating and an efficient catalyst.^[1] Ni composite coatings are self-lubricating materials for high-temperature applications.^[7] The nanofiller TiO₂ improves the mechanical and corrosion resistance.^[10] Specific Ni alloys, bright Ni-Rhenium (Ni-Re)^[40] and electroless deposited nickel-phosphorus (Ni-P) black coatings have distinct physical and electrochemical characteristics.^[6] Ni-Co alloys are utilized for sensitive bio-applications, such as protein microarray chips.^[9] Some Ni alloys fail due to high-temperature corrosion in atmospheres containing gases: sulfur, chlorine, and water vapor.^[13,15] Electrochemical noise monitoring is used to track sulfur corrosion in alloys like Inconel 718.^[14] Ni thin films and foams are used as anode and cathode current collectors in Li-air batteries.^[8] The Ni coatings improve the quality of laser-welded joints between dissimilar steels to prevent galvanic corrosion.^[12]

Zinc ED in the presence of additives

ED Zn and Zn-Ni are used in various industries, including automotive and aerospace, for the corrosion protection of steel components as sacrificial coatings. Conventional ED in different aqueous acid and alkaline baths suffered from HER, low CCE, and pollution. ED from non-aqueous ILs improved ED at an industrial scale.^[50]

The major Zn and Zn-Ni coatings are sacrificial anticorrosive protective coatings for steel in cathodic protection. Zn-Ni alloy coatings are more corrosion-resistant than pure Zn and serve as a good alternative to toxic cadmium (Cd) for protecting high-strength steel.^[25-29] The Ni content is the primary factor determining the physical characteristics and corrosion resistance of the coatings.^[25,27] The additives improve the microstructure, performance, texture, and surface morphology.^[18] Nanocrystalline zinc coatings exhibit better corrosion resistance.^[21] Organic additives, such as coumarin and polyethoxylated compounds, affected the ED mechanism and improved coating quality from acidic solutions.^[23,31] The nanodiamond-structured Zn composite coatings exhibit strong bonding and high load-bearing capacity.^[30] ILs and deep eutectic solvents are used to achieve more sustainable Zn and Zn-Ni ED.^[32,50]

In the absence of organic surfactant molecules, ED Zn was heterogeneous by adsorbed evolved H_2(g) molecules. The adsorbed sodium dodecyl sulfate (SDS) anionic surfactant formed uniform Zn coating crystals with pyramidal morphology. CTAB formed porous needles of Zn deposit crystals by hindering the approach of Zn ions and crystal growth. Triton X-100 gave cauliflower clusters plus ZnO. The high overpotential hindered the complete reduction to Zn.^[51]

Combined additives, including cetyl-tri-CH₃-NH₄-Br (CTAB), benzoic acid (BA), and 2-Br-3-Cl-5,5-di-CH³-cyclohex-2-enone (BCD), developed a bright white Zn coating on mild steel. The combinations: CTAB-BA and CTAB-BCD refined grain size and promoted preferred (100) and (110) crystal orientations. CTAB-BCD improved fine-grain size and brightness but developed macro pores and decreased the corrosion resistance. CATB and CTAB-BA exhibited higher corrosion resistance than Zn, primarily due to their smaller grain size. The lower corrosion resistance of optimized coatings is caused by surface roughness. CTAB, BCD, BA in the optimized bath (95% CCE, 18% T.P) produced a bright Zn nm crystal across a wide J range with 78% reflectance. The interaction of the additives with deposits enhanced grain refining and reduced surface roughness to 30 nm. Crystal aligned along the low-index crystalline (100) and (110) planes. High corrosion resistance is achieved by a smooth, uniform, non-porous insulating coating. The presence or absence of an individual or mixed additive decreased the corrosion resistance. Deposits at 0.25 mM to 1.25 mM in the optimized bath produced a good FE of 93-96% and T.P. of 13-18%. In addition, the coatings were smooth, uniform, bright, and oriented along the low-index (100) and (110) crystal lattice planes. At 0.5, 0.75, and 1.0 mM BCD, excellent corrosion resistance was achieved.^[52]

The glucomate ions enhanced the microhardness and adhesion of smooth ED Zn coatings on steel in an aqueous acidic ZnSO ⁴ solution containing mono sodium glucomate (MSG). Potentiodynamic cathodic polarization, cyclic voltammetry, in-situ anodic linear sweep voltammetry (ALSV), and kinetic Tafel parameters revealed that MSG accelerated ED via cathode depolarization, decreasing both normal overpotential and deposition potential. Chronoamperometry indicated that crystal growth was under diffusion control. pXRD patterns and SEM micrographs showed that MSG did not alter crystallinity but changed the crystal orientation of lattice planes and increased the hardness by threefold.^[53] In ED Zn on steel, ninhydrin ions in ZnSO₄ plating solutions formed fine-grained deposits. Polarization curves are cathodically shifted. Cyclic voltammetry indicated that ninhydrin increased nucleation overpotential and hindered ED. Initial nucleation stages and growth followed a 3D instantaneous nucleation mechanism.

The T.P. and the throwing index (T.I.) are enhanced by the synergism Nin-I^- ion.^[54] Epoxy resin particles did not affect ED but altered surface morphology and topology. These smooth, uniform, regularly arrayed compact coatings achieve excellent corrosion resistance. Polymer chains limited crystal growth and catalyzed crystal nucleation. Coating surfaces with larger polymer diameters was rough. Atomic force microscopy (AFM) suggested that smaller resin particle sizes reduced the surface roughness. The locus of potentiodynamic polarization curves remained unchanged, confirming the same polarization behavior and corrosion mechanisms.^[55]

Pure Zn and Zn-WO₃ composite coatings were electrodeposited on mild steel. A WO₃ loading of 1.0 g∙L⁻¹ improved the electrodeposited layer, as WO₃ particles were incorporated into the Zn matrix and refined the grain size, as shown by SEM micrographs. WO₃ particles increased Zn micro-hardness by 53.8%, which further improved at high concentration. The corrosion rate (CR) of Zn-coated steel is decreased by protective WO₃ barrier film composite coatings, which are low-cost, corrosion-resistant, and durable, making them recommended for steel tanks, containers, and boilers that require superior corrosion protection.^[56]

Zn coatings were electrodeposited on mild steel using various aqueous electrolytes, including halide-free alkali solutions, chloride-based DESs, and acetate-based organic solutions, for 30 min. The comparative corrosion resistance calculated from Tafel plots, crystal-nucleation, and growth revealed that the electrolyte type affected ED. The eco-friendly acetate solution exhibited the highest CCE, improved ED, and extended the electrolysis time from 15 to 30 min. The pXRD patterns indicated that various crystallite sizes and structures depend on the electrolyte type. Energy-dispersive spectroscopy (EDS) confirmed the purity of the deposited films. Polarization tests demonstrated that acetate enhanced corrosion resistance comparable to or better than the ILs.^[57]

ED of Zn on steel in an acid chloride bath with condensed products of 3, 4, 5-trimethoxy-benzaldehyde and eco-friendly chitosan polymer. The electrolyte bath, pH, current density (J), and temperature all affected the coating quality. Optimized bath composition and operational parameters affected the adhesion, ductility, and corrosion resistance. ED in Hull cell confirmed good T.P. and high CCE under different plating conditions. SEM photomicrographs clarified fine surface morphology. Additives formed adherent, bright, and corrosion-resistant coating.^[58] Certain conductive, low toxic and inexpensive ILs, which are prepared from substituted quaternary ammonium (NH₄⁺) salts and metal salts, influenced the reduction of metal ions during ED of alloys, and improved the deposit morphology. The ED anticorrosive coating, free from cracks, suggested scale-up metal finishing applications with high CCE.^[59]

Adherent smooth Zn ED on steel using aqueous MSG-SO₄-baths. MSG enhanced microhardness, and voltammetry confirmed that it decreased cathodic overpotential (αc) to a noble value, thereby depolarizing the cathode surface. Chronoamperometry indicated crystal growth under diffusion control. pXRD and SEM results showed that MSG unchanged surface morphology and a strong influence on crystallographic orientation.^[60] ED Zn coating on steel altered by polyethylene glycol (PEG 20000: molecular formula C_2nH_4n+2O_n+1, Mw. 20000 Da) that modified the cathode surface. Nucleation was under a 3D diffusion control mechanism. SEM micrographs illustrated that PEG altered the coating morphology to form hexagonal crystals perpendicular to the cathode surface, thereby smoothing the surface roughness from 70.0 nm to 29.8 nm in the deposits.^[61]

Pulse ED formed compact, shiny nm Zn deposit crystals (in non-cyanide alkaline-PEG electrolyte bath), with an average grain size range 30-50 nm. Corrosion resistance varied based on plating conditions. Corrosion current density (i_corr.) decreased with increasing off time. Both the peak current and corrosion current density in the presence of PEG increased as the pulse on time passes. The nano-crystalline Zn coating was bright, dense, and uniform, exhibiting the best corrosion resistance. Particle size decreased by PEG (reducing and capping protective agent for ED ZnNPs).^[62]

Various morphologies of Zn films ED using air-stable ILs. 1-butyl-1-CH₃-pyrrolidinium-tri-F-CH₃-SO₃H [Py1, 4] TfO; 1-ethyl-3-CH₃-imidazolium tri-F-CH₃- SO₃H [EMIm] TfO and their water mixtures. Cyclic voltammograms on Au electrodes displayed a characteristic redox process of Zn deposition and stripping in the same electrolyte. A shiny adherent and loose dark grey Zn deposit resulted from [EMIm] TfO and [Py1, 4] TfO, respectively. Both temperature and water significantly affected the morphology of the deposits and their electrochemical behavior. Pure Zn ED in water. Both Zn and Zn-Au alloys formed in IL, where temperature affected grain size, preferred grain orientation, and formed nanocrystal deposits with the pyrrolidinium ions.^[63]

The appearance of Zn ED from ZnCl₂ in ILs choline (urea or EG) is influenced by crystal nucleation and growth rates. During Zn crystallization, the nucleation rate was faster than bulk growth in urea and slower in EG IL. Electrochemical acoustic impedance parameters revealed that a critical surface coverage (θ_critical) shifted nucleation to bulk growth. It was similar for both IL, as it depends only on the substrate.^[64] In ZnED from SO₄-tartrate ions baths on vitreous carbon electrode, tartrate influenced the reduction kinetics and mechanism of ED. Potentiodynamic, potentiostatic, and voltammetry techniques indicated that tartrate decreased the reduction potential. A set of equilibrium represented electrochemically pH or potential-dependent reversible reactions. Chronoamperometry transients suggested that the initial deposition stages involved an instantaneous nucleation accompanied by 2D crystal growth. SEM imaging recommended.^[65]

In Zn ED on AZ31Mg alloy using ZnF₂ dissolved in NH₄-citrate solution, SEM images showed smooth, dense interface morphology. An adherent, stable coating resisted corrosion in NaCl for immersion times longer than 5 h. Cyclic voltammetry and polarization curves explored an activation-controlled ED mechanism affected by Zn(II) ion in the bulk electrolyte. Three Zn complexes contributed to electroplating, with an increased percentage of ZnL complex at high pH.^[66]

Temperature and J affected Zn ED from ZnSO₄ and [BMIM]HSO₄ IL. High temperature decreased the average crystal size, enhancing CCE and lowering energy consumption. IL decreased CCE by retarding crystallization and promoting Zn dissolution. Increasing J refined the deposits’ surface. The lower J increased CCE and decreased consumed energy, which was significantly above 500 A.m^-2. Cathodic polarization of Zn increased at high [IL] but decreased at high temperature. ILs inhibited the reduction reaction of Zn²⁺ ions. Potentiodynamic polarization measurements showed good agreement with the estimated kinetic parameters. For the Zn(II)/ZnO reaction, the cathodic transfer coefficient (αc) slightly decreased at high [IL] concentration but remained unchanged at 0.49 with increasing temperature. Jo is influenced by temperature and IL, followed by equation 3:

Thermodynamic parameters suggested that chemisorbed IL molecules on the cathodic surface followed the Langmuir adsorption isotherm.^[67]

Combined PEG-SGA in ZnCl₂ solution formed high-quality bright 70% reflectance ED Zn: uniform, dense, and fine-grained (average size 52 nm) with preferred orientation along low (100) and (101) crystal planes indices. Additives enhanced CCE and Tafel behavior, demonstrating a significant 60% reduction in the corrosion rate (CR) by PEG-SGA compared to single or no additive. Synergism PEG-SGA improved the corrosion resistance. At an applied J range of 1-6 A∙dm^-2, T.P. increased from 30% to 59% with CCEs of 79-98% forming excellent corrosion-resistant deposits. At high J, the effective anticorrosive PEG-SGA-Zn coating decreased CR and increased both the Rct and the polarization resistance.^[68] Particle loading and optimized ED parameters affected the Zn coating. The ED time ranged from 10-30 min at 200 rpm and 30°C. New interfacial coating properties acquired between 0.6 and 1.0 V. The electrolyte nature affected the coating thickness. Coating density at fixed anode: cathode distance and exposed surface area controlled the ED weight in advanced surface metallic coatings.^[69]

ED of Zn-Ni alloys

Glycine ligand, triblock terpolymer (PPG-PEG-PPG), and PEG1000 suppressed the ED of Zn and Zn-Ni alloys from acidified ZnCl₂ under instantaneous nucleation. Electrolytic composition affected cyclic voltammograms and the ED mechanism, respectively.

In the absence of additives, a single cathodic wave corresponding to ZnCl₃⁻ species and a second cathodic reduction wave due to Zn(Gly)₂²⁺ were observed. Nyquist impedance (Z) plots of the metallic coating involved a capacitive semicircle at high frequencies and two inductive loops at medium and low frequencies, due to adsorbed intermediates (I.M.). Equivalent circuit-fitted Nyquist plots consisted of two capacitive half-circles separated by an inductive loop, corresponding to the electrolyte solution. The polymer mixture affected the reduction of Zn(II) ions by increasing Rct and enhancing adsorption. ED Zn-Ni alloys revealed a single reduction wave for ZnCl₃⁻, Ni(Gly)²⁺ species, and multiple anodic signals depending on electrolyte composition. The process involved a multiple-step 3D instantaneous nucleation that included adsorbed species. Glycine modified the impedance (Z) inductive loop, reflecting the first Ni(I) adsorbed intermediate species. The coating morphology at 14-19 wt.% Ni was affected by the electrolyte composition, although the same crystal lattice was maintained.

The preferential lattice orientation was changed depending on the type of electrolyte used.^[70] In ED Zn-Ni alloy films from DES, ethylene diamine tetra-acetic acid (EDTA) and NH⁴Cl affected the electrochemical behavior, composition, morphology, and corrosion resistance. Cyclic voltammetry revealed that EDTA enhanced Zn incorporation and refined grain sizes at high concentrations, while decreasing CCE. Conc. NH₄Cl refined grain size, reduced Zn content, and increased CCE. The corrosion resistance followed the order: NH₄Cl > EDTA> additive-free electrolytes.^[71]

The compatible, perfect ED Zn-Ni and nanocomposite Zn-Ni-WC coating in an eco-friendly bath solution protected steel structures against chemical/electrochemical corrosion in an industrial environment. WC nanoparticles were incorporated into the alloy matrix, as confirmed by SEM micrographs and EDX spectra. This incorporation produced smoother grains, decreased crystallite size (as shown by pXRD), and enhanced microhardness. CR measurement using potentiodynamic polarization curves and impedance showed improved corrosion resistance and durability of the ternary alloy coating.^[72] Chemical additives enhanced the characteristics and mechanical properties of ED Zn-Ni alloy on steel from acidified ZnSO₄. Small amounts of Sm₂O₃ NPs anodically shifted the polarization curve and deposition potential, catalyzing the reduction of both metal ions. The Ni content range is 7.07% to 25.45%, depending on the operational conditions. pXRD analysis revealed the ED alloy consisted of Zn and Ni₅Zn₂₁ phases. XPS analysis confirmed that Sm ²O ³ increased corrosion resistance and microhardness from 62.0 to 151.00 kgf∙mm^-2. Temperature, pH, and J affected the coating morphology.^[73]

Electrodeposited Zn-Ni coatings obtained in an alkaline bath (pH 13.6) containing DETA acted as anticorrosive layers protecting high-strength steel, providing an alternative to toxic cadmium. Ni content, structure, and morphology varied by altering the agitation speed and the current density (J). A single c-Ni₅Zn₂₁ phase electrodeposited layer was obtained at low current density (J). At high current density, both hexagonal Zn and cubic Ni₅Zn₂₁ phases were electrodeposited. The lower J increased compactness and improved morphology. The single-phase layer showed the best corrosion protection. Denser morphology (sacrificial protection and durability) was achieved at 15 mA∙cm^-2 and 800 rpm. Incorporated hydrogen content was lower than that of Cd-coated annealed 4340 steel.^[74]

In electrodeposited multilayer Zn-Ni coatings on 2024-T3 Al alloys, the substrates were carefully pretreated by zincate or phosphate anodizing to prevent spontaneous Al₂O₃ formation and improve adhesion. A higher Ni percentage increased surface smoothness due to a refined microstructure. In scratch tests, coatings made of 16% and 4% Ni are brittle and ductile, respectively, due to differences in hardness and Young’s modulus. Mechanical properties adjusted to improve adhesion.^[75] Chitosan (CT) affected ED Zn-Ni coatings, altering the ED potential through adsorption and reducing the CCE. Surface characteristics affected by Ni content. A composite biocidal coating resisted microbiology-induced corrosion by sulfate-reducing bacteria and E. coli.^[76]

Cathodic J, NH⁴Cl, and substrate type influence Ni content in ED coatings. The structure and morphology varied with J. Low value caused the deposition of a single-phase γ-Ni ⁵Zn ²¹ coating with a Ni content range of 12-14%. Polarization curves confirmed that the Zn ED is more sacrificial than Cu. Impedance results explored barrier properties and corrosion performance in 0.1M HCl. NH₄Cl improved the electrochemical properties and protection. The influence of J confirmed that deposits developed at -20 mA∙cm^-2 without NH⁴Cl demonstrated superior behavior compared to other conditions. A complex relationship exists between factors J, additive, and substrate, affecting morphology and coating performance.^[77]

ED Zn-Ni on mild steel using (Zn sulfate-sulfanilic acid (SA) bath at different compositions, J, pH, and temperatures. Polarization and impedance results showed that Zn coatings at 3.0 A dm^-2, with 3.9 Wt. % Ni is the best anticorrosive. XPS spectra revealed that the noble Ni did not corrode, while the active Zn initially did. SEM micrographs confirmed dendritic coatings grown under different J suggested that electro-crystallization occurred under a mass transport mechanism. Corrosion performance of Zn is independent of both Wt.% Ni and the deposit thickness. These hard coatings are recommended for automotive and defense applications.^[78]

Both regular and anomalous co-deposition of Zn-Ni coatings, galvanostatically deposited at 60°C, showed that the Ni content and cathodic potential increased sharply, while the CCE significantly decreased above 40°C. Additionally, the alloy composition changed suddenly due to accelerated HER at high cathodic potentials during ED. Dense morphology, single phase, and crack-free structure achieved at the temperature range 30-40°C.^[79]

A Zn-Ni-Fe₂O₃ composite coating with uniformly dispersed Fe₂O₃ particles was electrodeposited from a Zn-Ni solution. The EDS spectra confirmed the presence of Fe₂O₃ during ED, resulting in smaller grain sizes. Composite coating was bright, compact, and anticorrosive. Fe₂O₃ altered the preferred crystal orientation and provided a superior corrosion-resistant coating.^[80]

ED of Zn-Ni alloy modifies the cathode surface, improving wear resistance, soldering ability, thermal conductivity, and hardness. To deposit Zn and Ni alloy simultaneously at the cathode, the conditions in equations 4 and 5 should be satisfied.^[81]

ED Zn-Ni depends on E^o, metal ions (M⁺ⁿ) concentration, activation overvoltage (η) for ED, and each metal in the plating bath. Adjusted [M⁺ⁿ] improved ED. Zn-Ni coating improved the lifetime of Zn sacrificial coatings. Zn-Ni alloy coating is more common than other Zn alloy coatings, and it is the most corrosion-resistant Zn alloy. The ductility and corrosion resistance of coated steel are improved by annealing. Zn-Ni coating is recommended for connecting bolts, brake system components, fuel system components, hydraulic fluids, and the aluminum industry because it resists heat and UV radiation. Zn-Ni is an expensive coating, which is electroplated in acidic and alkaline baths with properties controlled by different experimental parameters.^[81]

Corrosion-resistant Zn-Ni coatings were electrodeposited from non-aqueous IL electrolytes composed of choline chloride/ethylene glycol (1:2 molar ratio) eutectic mixtures and pure EG solvents. Electrochemical behaviors investigated by cyclic voltammetry indicated that both Zn and Ni ED. Ni content tuned to 10-20% Wt. % range. The highest % protection to industrial steel was confirmed from potentiodynamic polarization. Coating characterization, including morphological and phase composition analysis, confirmed the metastable γ ZnNi phase observed in both solvents.^[82]

ILs ED additives

The Ni content was controlled in Zn-Ni alloy coatings deposited on carbon steel. Experiments regulating the water content in choline chloride/urea IL confirmed that the Ni content decreased from 96 wt.% to 4 wt.% as the H₂O concentration increased from 0 to 7 Wt.%.

Water significantly influenced the Ni content, current efficiency (CCE), surface morphology, phase structure, and corrosion resistance. A water content of 5 wt.% was found to produce the most protective coating.^[83] The additive 1,2-dimethyl-3-(trifluoromethyl)-1H-pyrazol-2-ium trifluoromethanesulfonate influenced potentiostatic Zn electrodeposition from the [EMIm][TfO] IL at various applied potentials. Cyclic voltammetry explained the electrochemical behavior. SEM and pXRD analyses confirmed that the IL did not affect the shape or position of the Zn reduction peak. However, it significantly influenced the stripping peak, as micro- and nanocrystalline deposits were formed in both the absence and presence of the ILs (above the threshold concentration). The IL effect is enhanced at high concentrations, and adsorbed molecules hinder Zn nucleation, improving the deposit morphology.^[84]

Co and Zn coatings were co-deposited from EMIC IL. The cyclic voltammogram of ZnCl²-IL-CoCl ² showed Zn²⁺/Zn reduction peaks at -250, 0, and 200 mV. PCA analysis of EDS spectra assigned peaks to potentiostatic Co ED, underpotential Zn deposition on Co coating, and co-deposited Co-Zn alloy, respectively.

Cobalt was initially electrodeposited at −0.1 V (galvanostatically at 85 µA∙cm⁻²), followed by underpotential Zn electrodeposition under diffusion control. For Zn²⁺/Zn: αc slightly decreased at high [ILs] but was limited at 0.485 with rising temperature. Jo depends on temperature and additive concentration. The adsorption data fitted to Langmuir’s adsorption isotherm. Thermodynamic parameters suggested chemisorption.^[85]

ED of Zn on mild steel from the ChCl/ZnCl₂ system (1:2 molar ratio), which forms a completely miscible liquid at room temperature, produced a smooth, thick grey-white deposit at 0.29 mA∙cm⁻². A white thin film was deposited at a current density of 0.21-0.29 mA∙cm⁻². A heterogeneous deposit was obtained below 0.21 mA∙cm⁻².

The CCE above 99% indicated an effective non-porous anticorrosive barrier coating at low energy consumption. Carefully controlled applied potential prevented the evolution of Cl_2(g) at the anode from oxidized Cl-zincate complexes at 2.0V.^[86] In Zn ED from ChCl/urea IL, a rice-grain-like morphology was displayed. ChCl/2EG-formed thin platelet-like structures with a planar face perpendicular to the electrode surface. Atomic nucleation into geometrically ordered unit cells within the IL occurs faster than the subsequent crystal growth. Nucleation in ChCl/2EG-IL is slower than bulk growth.

ILs, which are more viscous than molecular liquids such as water due to strong van der Waals forces between bulky cations, influence mass transfer of metal ions (Mⁿ⁺), metal speciation, interfacial structure, deposition characteristics, morphology, and reduction mechanisms and deposition rates. Cyclic voltammograms showed J decreased during ED in ChCl/urea due to high viscosity.^[87]

During the ED of Ni-Cu alloy in ChCl-EG IL without other additives, the reduction potentials of Ni(II) and Cu(II) ions differ. High temperatures accelerated the co-deposition of Ni and Cu and improved corrosion resistance. At 70°C, the composition of the surface coating Cu_0.81Ni_0.19 alloy phase showed bright, flat, and pinpoint particles with micromorphology. Corrosion resistance of Ni: Cu ratio 7:3 equivalent to commercial Monel alloy.^[88]

During ED, reduction reaction (Zn⁺²/Zn⁰ particles) in ZnCl₂/[EMIm][Cl] IL was attained in acidified different Wt.% ZnCl₂. SEM images confirmed that Zn morphology is controlled by adjusting the deposition temperature and potential. Zn ED from IL 1-butyl-3-CH₃-imidazolium-Cl-([BMIM]Cl) on steel showed that [ZnCl₂]: IL ratio, J, and temperature made coating surfaces matte and mostly heterogeneous. A high ZnCl ² content improved corrosion resistance at the optimum ZnCl ²: IL ratio of 2:3.^[89] ED Zn coatings from ZnCl₂-([BMIM]Cl) bath on steel influenced by ZnCl₂:IL ratio, J, and temperature. Zn ED in all tested baths under all parameters. The coating’s surface was matte and had heterogeneous morphology in most cases. More corrosion protection is attained at ZnCl₂ content higher than 2:3 Wt.% [ZnCl₂]:IL.^[90]

At a constant ED potential of a bright Ni-Cu alloy coating on Cu from a ChCl-EG IL, cyclic voltammetry and chronoamperometry explored the nucleation/growth mechanism of metal ions. Kinetic parameters, polarization potential, and impedance were evaluated for the corrosion resistance coatings before and after passivation. At 70°C, reduction peaks aligned. High temperatures decreased the difference in reduction potentials of Ni/Cu ions. A 3D continuous nucleation/growth mechanism formed a velvety microstructure with morphology affected by ED potential and electrolyte composition. Dense velvety structure ED coating 16.95μm thickness formed at -0.85V (vs. Ag/AgCl_(s)/KCl_(sat.) reference electrode), was superior corrosion resistance (i_corr. 12.62 μAcm^-2, Rct 4.486 kΩ∙cm^-2). Passive CuO, Cu₂O, and Ni(OH)₂, enhanced corrosion resistance (i_corr. 2.107 μA∙cm^-2, Rct 16.91 kΩ∙cm^-2).^[91] ILs are excellent electrical conductors and strong electrolytes, less toxic than harmful solvents and dissolved metal salts, which can be deposited.^[92]

Coordinated water in NiCl₂.6H₂O decreased viscosity, improved the electrical conductivity of ChCl/2Urea eco-friendly, cheap IL, and promoted the formation of Ni nanocrystals without changing nucleation mechanism (progressive 3D nucleation with hemispherical diffusion-control growth) at high deposition potential. At 8 Wt. % H₂O, compact coating with 100% CCE electrodeposited from IL-NiCl₂ electrolyte. Hydrated Ni salts could replace anhydrous counterparts in high-quality ED Ni from IL, which is recommended for ED of another hydrated metal salt.^[93] IL [BMIM]HSO₄ affected CCE surface morphology, and crystallographic orientations during Zn ED from acidified ZnSO₄ solutions contained various dopant impurities, Cu, Fe, Co, Ni, and Pb. All dopants increased CCE except Pb (decreased CCE by 2%). Added dopants-IL electrolytes increase by 1-11%. Impurity contents in Zn deposits increased at high [impurity] in electrolytes, followed by the order: Cu < Pb < Fe < Ni < Co, and retarded by IL.^[94]

ED Co, Zn, and Co-Zn alloy on Cu, using an IL below -1.4 V, showed that metal-modified electrodes at -1.5 V for 15 min exhibited nm-scale micro-surfaces. The alloy exhibited agglomerated nano- and micro-sized particles. All coatings were crystalline except Cu (crystallinity decreased after ED)—corrosion measurements in 3.5 wt. The percentage NaCl solution showed i_corr. (μA∙cm^-2) decreased as: bare Cu (40.7) > Zn (8.91) > Co (3.89) > Co-Zn alloy (1.26).

The IL increased the corrosion resistance of Co-coated Cu by more than 30 times compared with uncoated Cu.^[95] In ED Zn coating: additives in Zn solution, nicotinic acid (NA), boric acid (BA), and benzoquinone (BQ) in ChCl:2 EG (DES). NA: Small-molecule additives improved brightness, decreased roughness, thickness, hardness, and corrosion resistance, the results of cyclic voltammetry, chrono (coulometry, amperometry), and microgravimetry. Redox peak currents and total charge decreased with the addition of ILs.^[96]

Zn ED in the absence of an additive is in good agreement with an instantaneous growth mechanism at short experimental time scales (indeterminate over more extended periods). In contrast, a progressive growth mechanism predominated when additives were present.

The current efficiency (CCE) of 95% obtained in DES electrolytes without additives decreased upon their addition. The resultant surface morphologies, thickness, topography, roughness, and crystal structure of the Zn coating, as revealed by SEM, AFM, and XRD, confirmed that the additives are effective brighteners, forming a uniform deposit. In ED Zn, Ni, and Zn-Ni alloys, pitting corrosion is avoided by anionic surfactant wetting agents, which decrease surface tension, residual stress, and eliminate hydrogen and air bubbles.^[97]

ED of metal-matrix composites (MMC)

Several electrodeposited metallic coating systems incorporate microscopic metallic particles, such as Ni-Co-(Al₂O₃/CeO₂) and Ni-Co-WC composites. Metals such as Fe, Au, Cu, Cr, Co, Ni, and Pb alloys are co-deposited with second-phase reinforcing metallic or ceramic particles, including carbides (W, Ti, Si, Cr, Zr, Ni), oxides (Al, Ti, Zr, Cr), nitrides (Si, B, Ti), borides (C, Zr, Ti, Ni), and sulfides (Mo, W).^[98] Tungsten carbide (WC) is applied in cutting tools, rock drills, punches, and wear-resistant coatings. Microhardness measurements of the composite coatings decreased with increasing Mo content, which consequently increased surface roughness and cracking and reduced corrosion resistance.

Reinforcing lightweight metals such as Al, Mg, or Ti into a Ni matrix produces flexible supports for structural applications at elevated temperatures. Fine, uniformly dispersed microparticles within the MMC restrict grain growth, reduce plastic deformation, and increase hardness. For (Co-Ni) MMC, additives such as WC, Al₂O₃, and SiC enhance hardness, mechanical strength, corrosion resistance, wear resistance, and morphology, except in acidic environments. Ni-Co solid solutions enhanced high-temperature properties.

WC-dispersed phase particles exhibit superior physicochemical properties, chemical inertness, and thermal stability, and they modify the Ni coating surface morphology from a pyramidal to an open cauliflower-like structure. Cobalt improved the surface morphology because its larger atomic radius compared to tungsten enlarged the open structure and increased hardness. Synergistic effects of grain refinement and particle dispersion strengthen the electrodeposited thin film (TF).

A high WC concentration in the electrolyte decreased the Ni grain size, thereby increasing hardness and promoting more noble electrochemical behavior. The coating produced with 8 g∙L⁻¹ WC exhibited the best corrosion resistance, characterized by a more positive corrosion potential, lower corrosion current density (i_corr), and higher charge-transfer resistance (Rct). The MMC (Ni-Co-WC) corrodes in acid media because the cobalt oxide (CoO) layer dissolves, causing galvanic corrosion.^[99]

Corrosion control and factors affecting properties

ED of Ni, Zn, and Ni–Zn alloys protects the base metal against corrosion, oxidation, and wear. Specifically, Ni provides protection through passivation, whereas Zn acts by sacrificial anodic protection.

Electroplating baths are selected according to the required coating characteristics.^[100] Low-cost ED Ni baths are composed of simple Watts electrolytes (0.63 M NiSO₄·6H₂O, 0.09 M NiCl₂·6H₂O, and 0.3 M boric acid) used either with or without additives, with pH adjusted using HCl or NaOH. The electroplating bath typically contains a Ni(II) or Zn(II) salt is either a simple salt such as a sulfate or chloride, or a complex salt such as cyanide, along with supporting electrolytes (e.g., Na₂SO₄, (NH₄)₂SO₄) to increase the electrical conductivity of the solution. The mechanical and corrosion resistance of electrodeposited Ni are enhanced on anatase mesoporous TiO₂ nanolayers. These nanolayers exhibit a high dielectric constant (ε = 80) and chemical stability at low temperatures. Their high packing density accelerates electron transfer to Ni(II) ions and facilitates Ni adsorption on the cathode surface [Figure 3]. The predominant (101) crystallographic surface, obtained by cutting Ti₃₆O₇₂ nanocluster blocks, is more reactive than the minor (100)/(010) and (001) surfaces. The major (101) surface caused surface roughness of the cathode by creating grooves filled with ED Ni.^[10]

Close

Figure 3:

Surface morphology of ED Ni on copper sheet roughened by TiO₂NPs (carried out by authors).

Export to PPT

The deposition mechanism involved weak physisorption of TiO₂ (electrophoresis) to the negatively charged cathode surface. Adsorption is reinforced in the following steps. Further reduction of the Ni(II) ion yielded an adherent ED Ni composite (compact, uniform, with negligible macropores). The adsorbed inert TiONPs deposited on the cathode surface prevented the development of metal crystallites. SEM revealed a good adherent surface film.

Organic additives enhance the physical and mechanical characteristics of the electrodeposited coatings, such as brightness, surface smoothness, thickness, strength, and corrosion resistance. Ascorbic acid, for instance, acts as a stabilizer and brightener that retards the oxidation rate of Ni. High concentrations decreased the ED rate, with surface roughness-boosted coating properties, and were used in ED high-quality Co-Ni alloys.^[100]

ILs improved the T.P. of the electrolyte bath and decreased the required electrical power more than complicated multivariable pulse ED (using the current waveform). Organic additives adsorbed at the cathode/solution interface via: physisorption, electrostatic attraction between charged metal (M) and charged additive molecules, dipole-type interaction between uncharged electrons (es) pairs in organic molecule with metal (M) surface, π-es bonds interaction with M, and all of these. The electron density of the additive molecules, the nature of the cathode surface, temperature, steric effects, multilayer adsorption, and surface-active sites^[101] enhance adsorption.

Many factors influence the composition and morphology of ED coatings. For example, the current density (J) affects adhesion, while electrolyte parameters such as composition, temperature, ion concentration, ionic strength, pH, and conductivity, together with electrolysis time, cathode properties, and agitation speed, jointly determine coating quality. All these factors affect the CCE, deposition rate, and microhardness of Ni and Zn, as well as their binary or ternary alloys.^[72]

Additives containing heteroatoms P, O, and S improved crystallinity. The weight of the ED film increased with increasing J, then leveled off. At low J, dissolved M⁺ⁿ slowly migrates and weakly adheres. Too high a J accelerated ion migration compared to diffusion (due to mechanical agitation), resulting in decreased particle co-deposition and coating hardness. At optimum J, all particles are loaded and incorporated in composite hard adherent coatings. Rising bath temperature from 30°C to 50°C affected the ED mechanism by increasing thermal motion and mass transfer and decreasing solution viscosity, hence improving dispersion and stabilizing the ED film. High particle content in the electrolyte film refines the grain size. Interfacial tension between ED composites and cathodic HER (accelerating desorption of electroplated particles from ED film) was reduced at 60°C. Above 70°C, water evaporated, resulting in a decrease in the stability of the ED coating film.^[102,103]

Most of the findings relate to optimizing Zn and Zn-Ni coatings for anti-corrosion applications, often using novel bath chemistries that provide microstructure control. Pulsed ED with surfactants/additives controls the morphology and orientation and achieves nanocrystalline Zn coatings with superior anti-corrosion properties.^[6,17,97] Additives for Brightness and Hardness: Specific additives and complexing agents, such as glutamate complexes,^[8,15] ninhydrin/iodide ions.^[9] Condensation products^[13] and tartaric acid^[20] improve the hardness, surface brightness, and anti-corrosion performance of the Zn deposit.^[7,23] Polyethylene glycol improves the deposition process.^[16]

ED Zn-WO₃ and Zn-Ni-WC nanocomposite coatings improve both the hardness and corrosion resistance of the steel substrate.^[11,27] Zn-Ni-Chitosan coatings found to provide microbial corrosion resistance and antibacterial properties.^[31] Composition and Morphology Control: Parameters like current density and deposition temperature influence the alloy composition, microstructure, and corrosion resistance.^{[22,32,34,35]} The nanoparticle additives, such as Sm₂O₃ catalyst, influence the deposition of the Zn-Ni alloy.^[28]

Non-aqueous, eco-friendly electrolytes such as ILs and DESs, often based on choline chloride, serve as alternative ambient-temperature media for the ED of Zn and Zn alloys,^{[19,37,38,41,43]} are new ambient temperature electrolytes for ED of Zn and Zn alloys,^{[14,18,19,37,41,44,45]} Ni,^[43,47,93] Ni-Cu alloys,^[39,46] Zn-Co alloys.^[95] DESs allow for the ED of composition-controllable Zn-Ni coatings^[38] and offer a promising sustainable route for the metal finishing industry.^[42] Additives are required even in the presence of DESs for further improvement of the coating quality.^[96]

Nanostructured and MMC coatings exhibit improved mechanical and physical properties due to the inclusion of reinforcing particles.^[98-104] Hybrid coatings, such as epoxy resin/Zn composites and natural polymer-based systems (e.g., chitosan), are used to enhance corrosion resistance and produce low-cost hydrophobic protective layers.^{[10,101,105,106]}

The selection of electrolyte (aqueous vs. IL/DES) has a profound impact on the final properties of the coating (Ref. 12). DESs, specifically, are reviewed for their protective performance against electrochemical corrosion.^[20]

Experimental ED setup and coating evaluation

ED was carried out in an electrolytic cell, schematically represented in Figure 4. The cell consisted of a rectangular transparent plastic trough (10.0 cm × 3.0 cm). Mercury (Hg) was used as an electrical conductor for the platinum anode.

Close

Figure 4:

Schematic representation of an industrial ED reactor (cathode Cu, Al, Fe, and St.St.) at a fixed position and appropriate distance from the Pt anode.^[103]

Export to PPT

Cu and St.St. Cathode surface cleaned by dipping 1.0 min. in a pickling cleaning agent 1.0M HNO₃, 1.0M HCl, respectively, to improve ED quantity and quality by creating surface grooves; washed, dried, and weighed. Removal of surface defects and coarse scratches was ensured by using the optical microscope. Each ED experiment began with a freshly cleaned cathode surface. After applying the required current density (J), the electrodeposited thin film (TF) coating was sealed in deionized water for 2 seconds and annealed at the appropriate temperature to improve crystallinity and quality by removing residual moisture and gas bubbles. The coated samples were then reweighed to calculate the Faradaic current efficiency (Eq. 6).^[104]

The actual electrodeposited mass is lower than the theoretical value (determined using Faraday’s laws of electrolysis) because of overpotentials and Ohmic losses. Highly conductive insulated copper cables were used in the external circuit. Their large cross-sectional area minimized Ohmic resistance and prevented voltage drop. The Ohmic overvoltage was calculated using equation 7.

Where ρ is the specific resistance of Cu wires (0.0175 Ω mm² m⁻¹), and l is the total length of the positive and negative leads. During ED, DC electrical cable losses were maintained below 1% of the applied power to ensure high current efficiency. The electrodeposited thin film was stabilized to enhance adhesion and mechanical strength using linear sweep voltammetry (LSV) at a scan rate of 0.1 V s⁻¹ for 25 cycles within the potential window of –0.8 V to –1.8 V.

High NiCl₂ concentration was used to eliminate residual internal stress expansion or compression of the electrodeposited TF. Boric acid acted as a buffer and improved the coating’s appearance. For laboratory-scale potentiostatic/galvanostatic ED and coating evaluation, a standard jacketed three-electrode cell was used. Cathode (WE), RE (Ag/AgCl/saturated KCl), and Pt counter electrode [Figure 5].

Close

Figure 5

Schematic design of ED in the Lab. Scale.

Export to PPT

The cells are connected to a computerized potentiostat. Certified analytical-grade reagents were used to avoid contamination from impurities such as hydrogen, sulfur, and other water-soluble elements. Mechanical agitation of the electrolyte increased mass transfer and decreased diffusion layer thickness, enhancing ion diffusion to the cathode. Above 50 rpm was avoided to eliminate disruption and leaching from the cathode surface due to turbulent flow. Solution de-aerated by bubbling nitrogen gas to eliminate interference from the oxygen reduction reaction. Bulk-scale platinum is a suitable inert, good conductor anode.^[105]

Cathodes of certified composition and known bare surface area were wetted and sequentially polished with emery papers of different grades, coarse (320, 400) and fine (600, 800), to achieve a mirror-like finish, then washed with deionized water and air-dried. Each ED experiment was conducted with a new, polished sample. The cathode surface was pre-treated to ensure a uniform and adherent coating. Solid particles and scales are removed mechanically by blasting and ultrasonic cleaning.

A Haring–Blum cell containing one anode between two cathodes was used to determine the % T.P., based on a near-to-far distance ratio of 1:5. Cathode surface catholically polarized by a 5 mVs^-1 sweep rate versus RE of constant known electrode potential.^[103]

Anodic linear sweep voltammetry (ALSV) was performed for Ni ED using a Pt working electrode (WE) and (WE) counter electrode (CE) at a fixed potential in the same plating bath. Stripping was then achieved by anodic polarization. The Platinum disc electrode cathode was polished using Al₂O₃ powder until a mirror-like appearance, washed with double-distilled water. Chronoamperometry (J-t transients) curves recorded.^[105]

The CCE was calculated based on the applied overpotential for the ED of Ni and Zn, which exhibit different electrochemical behaviors. Cell voltages for industrial ED computed using equation 8:^[103]

Where E_c, E_a, ƞ, and IR_cell are thermodynamic potentials (for cathodic and anodic reactions, respectively), overpotentials, and Ohmic overvoltage (resistance of electrolyte and electrodes depend on the cell design). Overvoltage was minimized by de-passivating the anode surface using chloride salts and acid pickling the cathode to remove scales.

Voltages of the ED cell are negative, and the cell’s energy efficiency was represented by equation 9:^[103]

The suggested ED mechanism

The bulk electrolyte contains solvated metal ions surrounded by counter-ions forming diffuse ionic clouds. Near the cathode, three interfacial regions are identified: a convective layer (<1 mm), a diffusion layer (∼100 µm), and an EDL, a few nanometers thick, where adsorbed particles dominate. Dispersed solid particles undergo constant Brownian motion in the solution.

When two particles approach closely, their behavior depends on interaction energy: if attraction exceeds repulsion, agglomeration occurs; otherwise, they remain separated. At repulsion forces higher than attraction forces, particle separation occurred. The magnitude of the summation force affects agglomeration (structures determined by system nature) and conditions such as J.

Weak van der Waals forces promote reversible adsorption of electrodeposited species and coexist in equilibrium with suspended particles in the electrolyte. Electrostatic (Coulombic) forces enhance the adsorption of charged species on the cathode surface. Some ions are trapped within surface pores, and the rate of mass transfer from the bulk solution to the cathode surface is the controlling step. Mechanical agitation enhances the transport of dispersed particles, while electrophoresis drives charged particles toward the cathode–solution interface.^[105-107] The particle concentration in the electrodeposit increases with additive content until all cathodic active sites are occupied and current limitation occurs. Excessive particle loading, however, promotes agglomeration and bath heterogeneity.^[106] In ED, a strongly acidic pH (∼2) is avoided to eliminate hydrogen evolution produces gas bubbles that damage the coating and hinder particle adsorption. An optimum pH range of 3-4 refines the deposited grains without decreasing the ED rate.

At pH values above 4, intermediate Ni(OH)⁺ species form and adsorb on the surface, which inhibits nucleation and growth of nickel particles. These intermediates suppress particle adsorption and nucleation rate but promote crystal growth in the deposited film. Increasing agitation speed enhances convection and mass transfer, reducing the diffusion layer thickness and improving ion diffusion to the cathode surface. However, excessive agitation can cause particle detachment due to turbulence.^[108]

During ED, metal ions (Mⁿ⁺) are reduced to adatoms (M⁰) that migrate across the cathode surface until they reach energetically favorable sites. Other atoms of ED aggregate with the first produced nucleus of a new phase. Number of nuclei formed, grown parallel and/or perpendicular to the cathode surface.

After monolayer coverage, subsequent multilayers are deposited on the substrate. Their structure and adhesion depend on overpotential, nucleation rate, and electrocrystallization, as described in Equations (10-12).^[103]

Where No, A are the numbers of nucleation sites and nucleation constant, respectively.

Instantaneous nucleation predominated at high overpotential:

Assuming monoenergetic sites, breaks in structure, such as grain boundaries and dislocations, decrease energy. During the crystal growth phase, nuclei grow parallel to and/or perpendicular to the surface. If growth probability is equal in x, y, z Cartesian coordinates, hemispheres of radius r, a 2πr ² surface area is formed for kinetically controlled ED. The dependence of J-time transients (represented by equation 13 and Figure 6)

Close

Figure 6:

Represented chronoamperometry (J-t transients) curves.

Export to PPT

For instantaneous nucleation, the exponent n equals 0.5 under diffusion control and 2.0 under kinetic control. For progressive nucleation, n equals 1.5 and 3.0 under diffusion and kinetic control, respectively. The variation of current density with time (J–t curve) provides insights into the electrocrystallization mechanism. However, experimental observations typically reflect an average of multiple growth modes, not all of which are hemispherical. An applied short pulse provokes initial nucleation at a high negative potential. Applied potential and J affected the deposition rate and structure of deposits:

A low overpotential allows sufficient time for atoms to arrange into a well-ordered crystalline electrodeposit. During ED, cathodic J (mA cm^-2) increased with applied potential and was limited at a potential corresponding to (startup ED). Then J was limited after monolayer coverage of the ED metal.^[106] The current-potential variation during ED has been represented in Figure 7.

Close

Figure 7:

Represented E-J during ED.

Export to PPT

The absence of a residual current at a low cathodic potential confirmed the presence of a good conductive electrolyte bath. TF changed physically at J range from -120 mAcm^-2 to 0.0 mAcm^-2: (coarse deposits at low J due to slow ion migration, and fine particles attained by rapid ion migration. The greater particle content in the electrolyte film created finer grains. The rising portion of the J–t curve represents the diffusion current. At the limiting current density (J_lim), all ions reaching the cathode surface are reduced, forming dendritic electrodeposits while eliminating non-Faradaic reactions. All ions reached the cathode surface and were reduced and electrodeposited. Increasing the particle concentration in the electrolyte generally results in finer grains in the electrodeposited film.

Characterization and evaluation of electrodeposited coating

A SEM is used to examine the surface morphology at magnifications up to 1000×. Micrometer-scale features revealed the detailed surface morphology. The primary energetic electron beam interacts with the specimen surface, producing secondary and backscattered electrons as well as characteristic X-rays. The primary energetic electron beam interacts with the specimen surface, producing secondary and backscattered electrons as well as characteristic X-rays.

Energy-dispersive X-ray spectroscopy (EDX) is a non-destructive analytical technique used to determine the elemental composition of electrodeposited coatings in situ with minimal sample preparation. EDX unit attached to the SEM, giving a unique spectrum showing peaks corresponding to the elements that constitute the accurate sample composition. The EDX unit is attached to the SEM and provides spectra with peaks corresponding to the constituent elements of the sample. Electrodeposited nanoparticles were examined using a transmission electron microscope (TEM, JEM-1400 Plus, Japan).

Powder X-ray diffraction (XRD) is a non-destructive and rapid technique used to identify and characterize electrodeposited crystals by matching patterns with the ICDD database. It provides information on crystal phases, preferred orientations, particle size, strain, crystallinity, and structural defects. Peak intensities depend on the distribution of atoms within the crystal lattice. XRD measurements were carried out at ambient temperature using a Bruker D8 Advance diffractometer (Cu Kα radiation, λ = 1.541 Å, 40 kV). Data were collected over a 2θ range of 10-80° with a step size of 0.02° and a scan rate of 1° min⁻¹.

The average crystallite size was calculated using the Debye-Scherrer equation (equation 14).^[104,105]

In this equation, β and θ represent the full width at half maximum (FWHM) and the diffraction angle (in radians), respectively.

Because Zn is an active metal and Ni exhibits both active and passive behavior in the galvanic series (in seawater), the coatings were evaluated for corrosion resistance using a three-electrode electrochemical cell [Figure 5].

The corrosion rate (CR) was determined in a thermostated electrochemical cell connected to a potentiostat. The working electrode (WE) was allowed to stabilize for 15 minutes in the test solution until the steady-state open-circuit potential (E_OCP) was reached. Electrochemical impedance spectroscopy (EIS) was then recorded using a 10 mV AC signal over a frequency range of 0.1 Hz to 30 kHz. Nyquist impedance plots (Z-real vs. Z-imaginary) were nonlinearly fitted to an equivalent circuit model.

Linear polarization resistance (LPR) was measured by applying ±20 mV, and potentiodynamic polarization was performed at ±250 mV around E_corr, with a scan rate of 1.0 mV s^-1, starting from the cathodic potential. The corrosion current density (i_corr) was obtained by extrapolating the anodic and cathodic Tafel lines to their intersection at the corrosion potential.

The corrosion rate (CR) is directly proportional to i_corr and inversely proportional to Rct. The percentage protection (%P) of the coating was determined using Equation (15).^[104,105]

Where i₀ and Rct₀ represent the corrosion current density and charge-transfer resistance of the bare metal, and i and Rct represent those of the coated samples. Corrosion of Ni or Zn coating in acidic media proceeds via the partial reactions that are shown in equations (16-18):

The formation of a passive NiO film represents the initiation of passivation. Upon anodic polarization, a more protective oxide film (NiO_x) is formed on Ni.^[21]

The Corrosion of Zn coating is represented as in equations 19 and 20:

The alloying elements enhanced the overall performance of Ni-based coatings. In particular, molybdenum increased hardness, wear resistance, corrosion resistance, and electrocatalytic activity. Optimized ED is a simple, low-cost, efficient technique that improves properties. ED (mechanism, parameters, elemental content, and coating properties depend on ED parameters and CCE.

Comparative studies

The comparative studies of ED Ni and Zn are represented in Tables 1 and 2.

The key insights drawn from this study are summarized as follows: Optimized ED is a simple, cost-effective, and efficient technique for improving coating properties. The ED mechanism, elemental composition, and coating quality all depend on process parameters and current efficiency (CCE). Pulsed ED forms nanocrystalline nickel coatings with improved grain refinement and hardness.

Alloying during ED, such as in Ni-Mo, Ni-Mn, Zn-Ni, Zn-Co, and Ni-Cu systems, enhances mechanical strength, corrosion resistance, and microstructural integrity. Eco-friendly ILs and DESs are green alternatives to conventional electrolytes, offering better control over deposition. The effectiveness of additives is demonstrated by organic compounds such as ascorbic acid, nicotinic acid, thiourea, and coumarin, which improve coating brightness, grain structure, and corrosion resistance. Organic additives, such as ascorbic acid, nicotinic acid, thiourea, and coumarin, which improve brightness, grain structure, and corrosion resistance, can demonstrate the roles of additives. Heteroatoms (N, O, and S) with lone-pair electrons form coordinate covalent bonds with the electrodeposited Ni film by donating electrons to the vacant d-orbitals of nickel adatoms. Natural additives such as kermes dye and phytic acid improved the quality and corrosion resistance of the metallic coatings. Composite hybrid coatings are self-lubricating and exhibit excellent performance in high-temperature and highly corrosive environments.

Zinc sacrificial coatings are commonly employed for cathodic protection of structural alloys, including carbon steel. The metallic coating applied in batteries involves nickel thin films and foams as current collectors in Li-air batteries. In battery applications, nickel thin films and foams are used as current collectors in Li-air batteries. It is recommended that future studies focus on utilizing sustainable green chemicals such as DESs and natural biodegradable additives. The metallic composite coatings exhibit high mechanical strength, resistance to general corrosion, and antibacterial activity. Nickel and zinc coatings enhance the electrical and thermal conductivity, reflectivity, and durability of steel substrates. The study clarifies the fundamental electrochemical principles governing the ED of Ni, Zn, and Ni-Zn alloys, both with and without additives.